Eeh.. length of time doesn't really make you less than a beginner. I've been at learning japanese for over 4 years now, and I am -still- a beginner.

"Da" is a very beginner concept. It is quite simply the plain form (dictionary form) of "desu".

"Na" (sometimes 'naa') at the end of a sentence is an emotion marker.

Na is like ne, except 'na' isn't really looking for agreement from anyone like 'ne' is. Often used when speaking to one's self (even if there's other people nearby). So... 'ne' wants a response, but 'na/naa' does not.

Like ryuubu said, it adds emphasis.. indicates a strong feeling. The 'aaa' sound gets stretched out longer the more emphatic the speaker is.

-----

A question of my own regarding that sentence...

What is the 'shi' in 'da shi'? Is it the suffix that pertains to some sort of order of things (next, last, etc). If so, does it modify the meaning any? What if the same sentence was said without 'shi'?

~shi implies that there is more than whatever shi was attached to.
using that ex:
ki-iroi wa suki da shi, furuino mo suki da yo
I like the yellow one (and what's more), I like the old one as well.

Of course as with almost everything in Japanese, you don't need to add what is more, but that ~shi implies that there is more. It doesn't have to be only one thing that is more, you can tack on multiple ~shi statements.

「な」, or 「なぁ」 when used like 「ね」 is more commonly used by men. That's not to say that women can't use it, just that they usually don't because it's seen as strong and unfeminine.
「な」 is also used to describe nouns, this usage doesn't really have a speciffic meaning, it's just a grammar thing and has no connection to the sentence ender 「な」.